Pressure?

When you use flint and steel you strike them together at speed, unlike ferrocerium rods where you can just scrape. Some people prefer to move the stone, I prefer to move the steel, since the stone is razor-edged and I can envision bad cuts if I miss. You don't need a lot of impact force if you have significant impact speed. Remember that your goal here is to cut a piece of steel off against the stone, and the heat of that cutting is what ignites the shaving of metal. The smaller the piece of metal and the faster it's removed, the hotter your spark will be, so ideally you want to strike very quickly and very lightly. Anyway, hitting the stone too hard will chip it and damage the edge... a good sharp piece of stone will light dozens of fires if used gently.

I'm not really sure how to quantify the technique, it's very hands-on and muscular memory. Fast and gentle. Break an old file, round off the side, and whack it against a shard of broken quartz. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" /> You'll quickly get a feel for what works. Youtube has lots of videos of striking if you need a visual.